1. Field of the Invention
The present invention relates generally to digital video and still cameras and more specifically to improving the quality of images captured by a digital camera having CMOS image sensors.
2. Description of Related Art
Digital imaging sensors using complementary metal oxide semiconductor (CMOS) technology in video and still cameras are replacing imaging sensors using charge coupled devices (CCDs). When a CMOS sensor image array is exposed to light from a scene image, photons hit the image array and a charge is generated by each CMOS sensor in the image array. A portion of each charge is proportional to the photon rate from the scene image. A remaining portion of each charge is due to process inaccuracies which are manifested as a fixed pattern of noise in the image generated by the CMOS sensors. This noise degrades the quality of the sensed image.
Dark Fixed Pattern Noise (DFPN) is a result of photo-diode leakage current sensed by the CMOS sensors. DFPN occurs even when there is no light reaching the CMOS sensors. Hence, it is a set of errors inherent in the CMOS sensor image array's operation. Some value of DFPN is added to the charge generated due to the scene image at each individual pixel of the CMOS sensor image array. The DFPN at each pixel varies significantly as a function of operating temperature, exposure parameters, and pixel location within the image array. It is highly desirable to remove the DFPN from the sensed image to improve the quality of the image.
Cancellation of DFPN is achieved by capturing a "dark image" in close time proximity to the sensing of a scene image. The dark image is captured when no light is reaching the CMOS sensor image array. The dark image is also captured with identical exposure parameters as the scene image. The dark image data is then subtracted from the sensed scene image data to produce corrected scene image data. The corrected scene image data has none of the image degradation due to DFPN which is normally prevalent in images obtained by CMOS sensors.
An electromechanical shutter is typically employed in the acquisition of a dark image for use in DFPN cancellation for both still as well as video modes of operation when using CMOS sensors as the image acquisition device. The principal of DFPN cancellation is illustrated in FIGS. 1A and 1B. FIG. 1A shows a digital camera system 10 having an electromechanical shutter 12 controlled by a solenoid or motor 14. In normal operation, a scene image 16 passes through the shutter to be processed by other components of the camera. However, for dark image acquisition, the shutter 12 is closed at the direction of solenoid or motor 14 so no light passes through to the other components. Since no light is allowed to reach the CMOS sensors, CMOS sensor image array 18 senses the charges inherent in the operation of the CMOS sensors according to the current operating characteristics and stores these charges as DFPN in dark image storage 20. Each sensor of the CMOS sensor image array 18 has its own value of DFPN. The data stored in the dark image storage 20 is an array of noise values for the pixels of the sensed image. FIG. 1B shows how the acquired DFPN data is used to correct a sensed scene image. While the shutter 12 is open, the scene image 16 is sensed by CMOS sensor image array 18. At this point, the sensed scene image data includes DFPN. A dark image subtraction unit 22 is included in the camera to reference dark image storage 20 to obtain the previously stored DFPN data, and to subtract the DFPN data from the sensed scene image data to produce corrected scene image data. The subtraction of the DFPN data from the sensed scene image data provides a significant improvement in the signal to noise ratio (SNR) of the resulting image. The corrected scene image data is passed to other components (not shown) for further processing such as display to a user or recording on a storage medium.
Since DFPN changes with environmental temperature and sensor settings, in video applications the shutter 12 needs to be used frequently in order to gather currently applicable dark images. The activity of operating the shutter increases the power consumption of the camera as well as decreases the mean time between failures (MTBF) for the shutter 12 and the solenoid or motor 14. Additionally, if the camera is coupled to a personal computer (PC) through a bus interface such as the Universal Serial Bus (USB), the increased power consumption required for frequent shutter operation in many cases will exceed the power limits imposed by the bus interface. When this occurs, the camera must supplement its power supplies with additional resources such as batteries or other high cost, low power electromechanical devices.
Of course, acquisition of dark images and cancellation of DFPN may be omitted in a CMOS sensor-based digital camera, but this may result in an unacceptable degradation of image quality.
A method of acquiring dark images and cancellation of DFPN that overcomes the disadvantages of the prior art would be desirable, especially for CMOS sensor-based digital cameras tethered to a PC.